Known generators adaptable for use in aircraft engines include flux switch generators, permanent magnet generators, Lundell Permanent Magnet Generators, Wound Rotor Synchronous Generators, Homopolar Generators, Hybrid Homopolar Generators, Slave Regulated Generators and Switched Reluctance Machines. Switched reluctance (SR) machines conventionally have multiple poles on both the stator and rotor; that is, they are doubly salient. In switched reluctance machines, there is a concentrated winding on each of the stator poles, but no windings or magnets on the rotor. Each pair of diametrically opposed stator pole windings is connected in series or parallel to form an independent machine phase winding of the multiphase SR machine. Torque is produced by switching current in each machine phase winding in a predetermined sequence that is synchronized with angular position of the rotor, so that a magnetic force of attraction results between the rotor poles and stator poles. Current is switched off in each phase before the rotor poles rotate past the aligned position. The torque developed is independent of the direction of current flow, so that unidirectional current pulses synchronized with rotor movement can be applied to the stator pole windings by an inverter using unidirectional current switching elements, such as transistors or thyristors. For use as a generator, the current pulses in each machine phase winding are simply shifted so that current flows when the rotor poles are moving past alignment towards the unaligned position.
A SR motor drive or generator system operates by switching the machine phase currents on and off in synchronism with rotor position. That is, by properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained. Usually, the desired phase current commutation is achieved by feeding back a rotor position signal to a controller from a shaft angle transducer, e.g. an encoder or a resolver. However, in order to reduce size, weight and cost in SR motor drives and generating systems, techniques for indirect rotor position sensing have been developed, thus eliminating the need for a shaft angle transducer. Current regulators are typically employed for controlling phase current amplitudes in a SR machine.
Control Alternators used in present day aircraft engines are normally driven by the engine shaft and, as a result the output may be subject to voltage variations of 10 to 1. Therefore, voltage from the permanent magnet generators used for this purpose can not be regulated easily. If an SCR controlled rectifier system is used to maintain constant output voltage from 10% speed to 105% speed, the rectifiers must be large enough to block the high voltages generated at top engine speed. Further, a failure in the controlled rectifier may expose the rest of the electrical circuitry to excessive voltages.
Alternatively, a "shunt regulator" may be used at high speeds to short the alternator such that only a percentage (e.g., 10%) of the generated voltage reaches the electrical circuitry. A failure in the solid state switches of a shunt regulator (controlled rectifier) does not expose the other circuitry to high voltages because the alternator output is effectively shorted. Therefore, the alternator must be designed to handle continuous short circuit operation in the event of a failure.
It would, therefore, be advantageous to design a switched reluctance machine which does not expose the low voltage electronic circuitry to high voltages in the event of a failure, nor does it expose the generator to continuous short circuit operation and excessive temperatures.
It will also be noted that, starting a conventional switched reluctance generator normally requires a battery or other power source to energize the phase windings.